The present invention relates to a method for writing data of a file on a tape medium (or magnetic tape unit) so that the data can be accessed at high speed. More particularly, the present invention relates to a writing method that enables high speed reading of data on an index partition of a linear tape file system (LTFS).
The LTFS is a file system that realizes direct access to a fifth generation linear tape open (LTO), i.e. LTO-5, tape drive and an enterprise tape drive (e.g., IBM TS1140), as well as a tape library that uses these tape drives. According to the LTFS format, an LTO partition function is utilized to divide a tape (also referred to as a tape medium, a medium, a tape cartridge, or simply a tape herein) into two partitions. The two partitions are an index partition (IP) which is a region in which metadata is written, and a data partition (DP) which is a region in which the data body is written. The LTFS is a method that utilizes a tape medium, via a file system, as if the tape medium were a hard disk drive (HDD) or a Universal Serial Bus (USB) memory. The LTFS stores the body of a file on the DP of the tape medium, and stores metadata relating to the file, referred to as an “index”, in the XML file format on the IP.
FIG. 1 illustrates a tape medium that has been divided into two partitions according to the conventional LTFS. An LTO-5 tape drive writes data while causing a write head to make a round trip over wraps of a write region along the longitudinal direction of the tape medium. The LTFS utilizes two of the wraps (amount of one round trip) as an index partition. The tape medium is divided into two partitions that are used as the IP and the DP from a beginning of tape (BOT) to an end of tape (EOT). A wrap is equivalent to a track. A tape cartridge according to LTO-5 has a length of approximately 800 m in the longitudinal direction of the tape medium, and has a width of 80 wraps in the transverse direction. The IP and the DP are divided by a guard band composed of two wraps. The tape medium travels backward and forward in the longitudinal direction of the wraps. A situation where the travelling direction thereof is inverted at the BOT or the EOT is referred to as a “wrap turn”. It is possible to change the number of partitions to an arbitrary number that is greater than or equal to three in accordance with various specifications.
In some cases, in addition to metadata, user data of a specific format and size specified by the user is also written on the IP. This is referred to as a “rules function”, and is a function created with the expectation that access to specified data would be speeded up by writing the data in the particular area that is designated the IP. However, the following problems exist with regard to this function.
A first problem is that access to target data is not faster even when the rules function is used. The IP and DP are divided along the longitudinal direction of the tape. Consequently, even in the case of data written on the IP, a seek occurs in the longitudinal direction, similar to the case of data being written to the DP and being accessed from the DP. Since the time taken to access data depends on the seek time, it is not necessarily the case that an access will be faster even when data is written on the IP.
A second problem that is encountered is that mounting of the tape medium takes time if the rules function is used often. The LTFS is configured as a mechanism that reads metadata written in the IP for storage in a main memory at the time that a tape medium is mounted. Consequently, mounting processing does not end until reading of all metadata finishes. According to the LTFS format, metadata is written at the end of the index partition. In a case where a large amount of data is written to the IP by means of the rules function, a long seek time is required to reach the metadata, and consequently the time until mounting is completed lengthens.
The documents JP2007-286679A and JP2008-293189A describe storage systems in which one portion of a file is stored in a cache (primary storage) and the remaining portion is stored on a HDD (secondary storage).
However, the technology disclosed in the aforementioned documents does not enjoy the advantage of portability that is obtained by holding all the data of one file on a single tape cartridge.